Wacky planets force a rethink

Diagram showing the orbits of three planets around the star Upsilon Andromedae A

Three Jupiter-type planets orbit the star Upsilon Andromedae A. Astronomers have found that the orbits of two of the planets are inclined by 30 degrees with respect to each other, something that hasn't been detected in any other planetary system.

  • Planets orbiting a distant star
  • Their orbits are wildly tilted to each other
  • Forcing a rethink of planetary evolution

The discovery of a planetary system “out of whack,” where the orbits of two planets are at a steep angle to each other, has been reported today by a team of astronomers led by Barbara McArthur of The University of Texas at Austin McDonald Observatory.

The surprising finding will affect concepts of how multi-planet systems evolve, and shows that some violent events can happen to disrupt planets’ orbits after a planetary system forms, say the researchers.

“The findings mean that future studies of exoplanetary systems will be more complicated. Astronomers can no longer assume all planets orbit their parent star in a single plane,” McArthur says. (An exoplanet is one that orbits a star other than the Sun.)

McArthur and her team used data from Hubble Space Telescope (HST), the giant Hobby-Eberly Telescope, and other ground-based telescopes combined with extensive computer modelling to unearth a landslide of information about the planetary system surrounding the nearby star Upsilon Andromedae (“Ups And”).

McArthur reported these findings in a press conference at the 216th meeting of the American Astronomical Society in Miami, along with her collaborator Fritz Benedict, also of McDonald Observatory, and team member Rory Barnes of the University of Washington. The work also will be published in the June 1 edition of the Astrophysical Journal.

A new angle on the theory

For just over a decade, astronomers have known that three Jupiter-type planets orbit the yellow-white dwarf star Upsilon Andromedae. Similar to our Sun, Upsilon Andromedae lies about 44 light-years away. It’s a bit younger, a bit more massive, and a bit brighter than the Sun.

Diagram showing a comparison between our Solar System and the Upsilon Andromedae A system.

A comparison of planetary orbits in our Solar System and the Upsilon Andromedae A system.

Much more startling, though, is the finding that not all planets orbit this star in the same plane. The orbits of planets “c” and “d” are inclined by 30 degrees with respect to each other.

This research marks the first time that the “mutual inclination” of two planets orbiting another star has been measured. And, the team has uncovered hints that a fourth planet, “e”, orbits the star much farther out.

“Most probably Upsilon Andromedae had the same formation process as our own Solar System, although there could have been differences in the late formation that seeded this divergent evolution,” McArthur said.

Until now the conventional wisdom has been that a big cloud of gas collapses down to form a star. Left over material forms a flattened cloud—known as a “disc”—surrounding the young star, and the formation of planets within it is a natural by-product. In our Solar System, there’s a telltale sign of that process because all of the eight major planets orbit in nearly the same plane.

“But now we have measured a significant angle between these planets that indicates this isn’t always the case,” says McArthur.

On the precipice of stability

So how did the two planets end up in such dissimilar orbits?

Possibilities include gravitational distortions during close encounters between planets in the system. Or it could have been a similar gravitational disruption caused by the parent star’s binary companion star, Upsilon Andromedae B.

The astronomers don’t know which scenario is correct, but they’ve found that the current orbital configuration is “right on the precipice of stability.”

“The planets pull on each other so strongly that they are almost able to throw each other out of the system,” says Barnes.

The team has also uncovered hints that a fourth, long-period planet may orbit beyond the three now known. There are only hints about that planet because it’s so far out, the signal it creates does not yet reveal the curvature of an orbit. Another missing piece of the puzzle is the inclination of the innermost planet b, which would require precision measurements 1,000 times greater than Hubble’s, a goal NASA’s planned Space Interferometry Mission (SIM) could attain.

The team’s Hubble data also confirmed Upsilon Andromedae’s status as a binary star. The companion star is a red dwarf less massive and much dimmer than the Sun.

We don’t have any idea what its orbit is,” Benedict said. “It could be very eccentric. Maybe it comes in very close every once in a while. It may take 10,000 years.”

Such a close pass by the primary star could gravitationally affect the orbits of its planets.

Adapted from information issued by McDonald Observatory, The University of Texas at Austin / STScI / NASA / ESA / A. Feild (STScI) / B. McArthur (The University of Texas at Austin, McDonald Observatory).

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